Firefighters who have deployed fire shelters during training
have had difficulty communicating using hand-held radios. The Missoula
Technology and Development Center (MTDC) conducted a brief study to determine
how well radios worked inside fire shelters.

The study showed that when firefighters were inside fire
shelters within 50 feet of each other they could communicate using the
VHF (Very High Frequency, 30 to 300 MHz) Bendix-King radios. They could
not communicate using the newer UHF (Ultra High Frequency, 300 to 3,000
MHz) Motorola Astro XTS 3000 radios. In either case, the radio signals
were significantly weaker when the radio (figure 1)
was used inside the fire shelter, particularly when the radio was inside
the New Generation Fire Shelter.

Essentially, firefighters could shout and be heard as
far as if they used their new UHF radios inside a fire shelter. Firefighters
probably won't be able to make effective use of their hand-held
radios when they are trapped inside their fire shelters. Once the fire
has passed, and it's safe to leave the shelter, firefighters can
use their hand-held radios to establish communications with each other
and with their supervisors.

Study Details

Various factors influence how effective radio communications
will be from inside a fire shelter. Those factors include the location
of the fire shelter relative to the radio receiving the signals, the location
of the radio inside the shelter, the orientation of the antenna, and the
fire shelter model. MTDC also looked at different types of radios and
antennas.

For the tests, a ¼-wave vertical whip antenna
was used to receive VHF signals at 168 MHz. A log-periodic Yagi antenna
was used to receive UHF signals at 870 MHz. The strengths of VHF and UHF
signals were measured on a Rohde and Schwartz FSH3 spectrum analyzer (figure
2).

Figure 2—The log periodic Yagi antenna (left) and a spectrum analyzer
were used to determine signal strength from the UHF Motorola hand-held
radio.

Signal strengths were measured in decibels (dB) and decibels
referenced to 1 milliwatt of power (dBm). See the decibel refresher section
below for further explanation of these units of measure. Although the
UHF signals were about 5 dB stronger than the VHF signals because of the
different types of antennas used to receive the two signals, relative
difference in signal strengths did not depend on the type of antenna that
was used. The spectrum analyzer and antennas were placed 50 feet from
the fire shelters or other radio transmission points. The weather was
clear and sunny with a temperature of about 60 degrees F.

The VHF radios tested were Bendix-King GPH5102X models
set for 2 Watts (W) of RF (radio frequency) output. The antennas were 6-in helical
"rubber ducks." The UHF radios tested were Motorola Astro
XTS 3000 models with 3 W of RF output. The receiver sensitivity specifications
were similar for both types of radios, about 0.3 Microvolt (µV) for intelligible
reception.

For one test, a fire shelter was aligned with the path
to the receiver. An individual inside the fire shelter positioned himself
with his head at the end of the shelter nearest to the receiver. His feet
were at the other end. He first held the radio 6 inches from the end of
the shelter nearest to the receiver and then held the radio in the shelter's
center.

The radio was tested at three antenna angles: vertical,
45 degrees, and horizontal in the plane perpendicular to a line to the
receiving antenna. Although antenna polarization can be a major factor
in signal reception at VHF frequencies and above, the aluminized surface
of the fire shelter appeared to alter the polarization of the signal so
that the receiving radio's orientation wasn't critical.

An FSH3 spectrum analyzer with a 160 MHz ¼-wave antenna
50 ft from the center of a fire shelter was used to test the VHF Bendix-King
GPH5102x radios (set for 2 W output at 168 MHz). The same analyzer with
an 800 to 2,000 MHz log-periodic Yagi antenna 50 ft from the center of
a fire shelter was used to test the UHF Motorola Astro XTS 3000 radio
(3 W output at 870 MHz).

During the tests (table 1), the
received signals varied by several decibels because the radio's
position was adjusted inside the shelter. Therefore, the values in the
table should be considered approximate, within several decibels per milliwatt
of the precise values for the specified test conditions. The accumulated
signal strengths from three radio orientations in two locations within
the shelters were averaged to show the attenuation (reduction of strength)
more accurately.

Table
1—Test results showing how much transmissions from the VHF
Bendix King and UHF Motorola Astro hand-held radios were weakened when
the radios were used inside fire shelters. All values are in decibels.
The radios were held vertical, 45 degrees from vertical, or horizontal.

Radio type and antenna orientation

VHF vertical

VHF 45
degrees

VHF horizontal

UHF vertical

UHF 45
degrees

UHF horizontal

Shelter

Old shelter-center

47

44

44

58

59

59

Old shelter-end

63

59

63

58

59

65

New shelter-center

55

59

55

66

63

77

New shelter-end

60

58

57

60

58

63

No Shelter

No shelter

20

22

20

10

16

25

Signal attenuation relative
to no shelter

Old shelter-center

-27

-22

-24

-48

-43

-34

Old shelter-end

-43

-37

-43

-48

-43

-40

New shelter-center

-35

-37

-35

-56

-47

-52

New shelter-end

-40

-36

-37

-50

-42

-38

Based on the averages for each test setup, the attenuation
of radio signals transmitted from inside fire shelters was as follows:

Old shelter model with a VHF radio: -33 dB

Old shelter model with a UHF radio: -43 dB

New Generation Fire Shelter with a VHF radio: -37
dB

New Generation Fire Shelter with a UHF radio: -47
dB

Firefighters inside fire shelters also have difficulty
communicating over the radio to other firefighters inside fire shelters.
Another test was conducted to study this problem.

An individual positioned himself inside an old shelter
model. Another individual was in a New Generation Fire Shelter about 50
ft away. Communication was possible with the VHF radios, but not with
the UHF radios. According to the averaged signal strength measurements,
shelter-to-shelter signals for UHF radios would be -20 dB weaker
than VHF signals. This test indicates that communication from inside the
New Generation Fire Shelter is more difficult than communications from
inside the standard shelter.

Some firefighters have questioned whether they might
risk damaging their retinas if they use their hand-held radio inside a
fire shelter. The risk of using the radio inside the shelter is essentially
the same as that of using the radio elsewhere—negligible. The hand-held
radios operate at such low power that the risk is essentially eliminated.
The limited amount of energy that is reflected from the inner surface
of the shelter to a firefighter's head would be much less significant
than the energy transmitted from the antenna when the radio is held in
front of a firefighter's face. So, even though hand-held radios
don't work very well when they're used inside a fire shelter,
they don't present a risk to firefighters who try to use them there.

A Decibel Refresher

Radio technicians and engineers work extensively with
signal strengths measured in decibels because they can represent extremely
large or small values with two- or three-digit numbers. Decibels work
on a logarithmic principle. When working with signal power, the value
of a power gain or loss in a system is 10 times the logarithm of that
gain or loss. For example, a system with a power gain of 100 would have
a gain in decibels of 10 times the log of 100, or 10 • 2 = 20 dB.
The log of 100 is two because 102 = 100. Similarly, a circuit whose output
is 1/1,000 the input power has a gain in decibels of 10 • (-3) =
-30 dB. The log of 1/1,000 or .001 is -3 because 10–3 = .001.

At radio frequencies in the UHF region (300 to 3,000
MHz), signal power detected from a transmitter decreases by a factor of
1/4, or -6 dB, every time the distance to the transmitter doubles.
Increasing the distance 10 times results in a received power loss of 1/100,
or -20 dB.

This information refers to relative changes in signal
strength. It does not describe the signal power being transmitted or received.
When absolute power information is required, the unit found most often
in communications work is the dBm, or decibels referenced to 1 milliwatt
of signal power. A transmitter with an output power of 2 W would provide
a signal strength of 33 dBm at its antenna. Good VHF receivers can detect
signals in the range of -107 dBm with quarter-wave antennas; "rubber
duck" antennas reduce signal detection to the range of -103
dBm.

About the Author

Ted Etter joined MTDC
in 2002 to work on electronics projects. He has spent more than 25 years
working in the areas of electronic instrumentation and display technology.
He received a bachelor's degree in mathematics from the University
of Oregon in 1992 and a master's degree in teacher's education
from Eastern Oregon State University in 1993. Before coming to MTDC, he
taught courses in programming, digital circuits, data communications,
radio frequency communications, robotics, microprocessors, and operating
systems at the University of Montana College of Technology.